This invention relates to tungsten-188/rhenium-188 and molybdenum-99/technetium-99m generators, and more particularly to a process for their preparation.
Technetium-99m and rhenium-188 are important radionuclides, used in diagnostic and therapeutic applications in hospitals and other establishments. Several generators which separate the daughter radionuclide, technetium-99m, from its parent radionuclide, molybdenum-99, and the daughter radionuclide, rhenium-188, from its parent radionuclide, tungsten-188, have been described in the literature and/or have been commercially available.
Chromatographic generators, such as those used to produce Tc-99m from Mo-99, typically contain insolubilized parent radionuclide adsorbed onto a bed or column of material such as alumina for which the daughter has relatively little affinity. The daughter radionuclide, which forms from decay of the parent, is then periodically eluted from the column, for example, using physiological saline.
Many Tc-99m generators currently in use utilize Mo-99 produced by the fission of highly enriched U-235 targets. Fission Mo-99 has extremely high specific activity, i.e., &gt;10,000 Ci/gram. Multicurie amounts of Mo-99 can thus be adsorbed on very small alumina columns (i.e., 1-1.5 grams of alumina) which can be efficiently eluted to obtain high concentrations (i.e., &gt;1 Ci Tc-99m) in low volumes (i.e., less than 2-5 mL) of eluate. However, fission of U-235 results in the production of large quantities of gaseous and solid radioactive materials of many elements--a burdensome and costly waste management issue.
Although it is possible to produce Mo-99 via neutron bombardment of natural Mo-98 targets, this (.eta.,.gamma.) reaction produces low specific activity (e.g., approximately 2.5 Ci/gram) Mo-99. Generators made with such low specific activity Mo-99 require substantially larger columns which, in turn, require increased volumes of eluant. The resulting Tc-99m solution contains undesirably low concentrations of Tc-99m in large volumes.
In U.S. Pat. No. 4,280,053, Evans et al. describes a Tc-99m generator containing zirconium molybdate (ZrOMoO.sub.4) gel prepared from (.eta.,.gamma.) Mo-99. The gel is prepared by dissolving Mo-99 in a slight excess of aqueous ammonia or sodium hydroxide solution. Acid is added to adjust the pH to between 1.5 and 7 and the resultant solution is added to a stirred aqueous solution of zirconium. A molybdate precipitate is formed. The precipitate is collected by filtration or evaporation of the liquid, air-dried and then sized for use in a generator.
In U.S. Patent 4,859,431, Ehrhardt describes a process for the preparation of zirconium tungstate (ZrOWO.sub.4) gel generators. Irradiated tungsten trioxide is dissolved in a heated basic solution and added to an acidic zirconium-containing solution to form an acidic slurry in which a zirconyl tungsten precipitate forms. The slurry is neutralized using a basic solution, the precipitate is filtered, washed several times, dried, crushed and transferred to a generator column.
The processes described by Evans et al. and Ehrhardt for the preparation of zirconium molybdate gels and zirconium tungstate gels are not, however, without limitations. After the acidic slurry is formed, the pH must be adjusted, the slurry must be filtered and washed, and the dried precipitate must be crushed to the desired particle size. It is technically difficult to produce commercial quantities of highly radioactive zirconium molybdate gels and zirconium tungstate gels using these many and varied steps.